Warming up the underwater brood period

Enormous numbers of deep-sea octopus are congregating around the base of certain seamounts, scientists say. 

There are so many, in fact, that a team led by James Barry of the Monterey Bay Aquarium Research Institute, US, counted 7,242 nesting females in a single 2.5 hectare “Octopus Garden,” about 120 kilometres offshore from central California.

The octopuses, all of the single species Muusoctopus robustus, which Barry describes as about the size of a grapefruit, appear to be drawn to warm-water springs at the base of the seamount, huddling along the flows of the water emanating from them.

A massive colony of octopuses (Muusoctopus robustus) observed near Davidson Seamount in the Monterey Bay National Marine Sanctuary. These octopuses are likely brooding their eggs: protecting them by inverting their arms and covering them with their body. Credit: Ocean Exploration Trust and the National Marine Sanctuary Foundation.

These springs, says Ann Hartwell, a PhD candidate at the University of New Hampshire, US, who has studied not only this seamount but another offshore from Costa Rica, were only discovered in the last 10 years and appear to be very different from the much hotter geothermal vents that have fascinated ocean scientists for decades.

Those vents are super-hot, while the newly discovered ones are merely warm, raising the water temperature by at most 10°C, Hartwell told the recent Ocean Sciences Meeting. They appear to be created not by the type of volcanic processes that produce the superheated water of previously discovered hydrothermal vents, but by the circulation of seawater just deep enough below the surface to warm up a bit before coming back up.

Why octopuses are drawn to these warm springs was initially a mystery. But Barry thinks it’s because the warmth is just enough to help them reproduce more effectively.

At the depths of the Octopus Garden, 3200m below sea level, the normal temperature is 1.6°C, he says. That’s an issue for octopus reproduction, because studies have shown that the brood period of octopuses – meaning the time needed for their eggs to develop and hatch – is extremely long, requiring not just weeks or months, but years. It is also dramatically affected by water temperature. At 5°C their brood period is on the order of 600 days. At 1.6°C it is likely be pushing close to 14 years. 

Curious, Barry’s team used submersibles to study 26 nesting females, watching the rate at which their eggs developed. They also measured the water temperature at the places they were nesting, and found that it averaged 5.1°C. And, they found, these females’ brood periods averaged 576 days—a far cry from the 14 years they’d have spent in the cold waters nearby. No wonder they were drawn to the warmer waters.

Octopuses, Barry says, are probably not the only deep-sea species to have discovered the value of warm-water seeps, nor are the two known locations likely to be the only ones.

“There are probably many such aggregations of animals breeding in odd or undiscovered areas that we need to think about before we wholesale go out and start mining things or recklessly abandoning any concern for the systems we know so little about,” Barry says.

Studying these areas can also add vital information to our understanding of the global-warming future. 

Overall, the deep sea is warming at the rate of .01°C to .1°C per decade, Hartwell says. That means that studying biota close to but not immediately on the warmest parts of these deep-sea vents—where the temperature difference is on the same order as that expected from global warming, provides a perfect natural laboratory for peering into the future. “That can improve the accuracy of predictive models and inform management strategies,” she says.

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